They dissembled the negative washed it. And reassembled it with the new material. And the new optical recomposites. We were lead to believe all the Color Reversal Negative was unusable, perhaps it was back then.
What always got me was that no protection master was made of the restored print with none of the cgi changes. It probably would have been paid for by 20th Century Fox. It didn’t happen because of Lucas direct intervention, this 1997 cut is the original version.